Meryl Streep, Julie Walters, and Christine Baranski
The play itself was performed after Julie Walters managed to get permission to use the songs. The movie was filmed after a whole bunch of filming groups tried to get Julie Walters to give them permission to make the movie, but she was hard to convince because she wanted the Mamma Mia movie to still be her vision, not theirs.
For the 2008 movie the lead women were: Meryl Streep, Amanda Seyfreid, Christine Baransky, and Julie Walters.
Mamma Mia is set in Greece.
Mamma Mia starred multiple people. This list includes the actor as well as the character: Meryl Streep as Donna, Pierce Brosnan as Sam, Colin Firth as Harry, Stellan Skarsgard as Bill, Julie Walters as Rosie, Dominic Cooper as Sky, Amanda Seyfried as Sophie, and Christine Baranski as Tanya.
If you mean Amanda Seyfried who played the roleof Sophie then she was 23 years old in Mamma Mia!
. . .